Three premises are examined: (1) mechanical properties of arteries are determined by the relative amounts of elastin and collagen and their degree of crosslinking; (2) amounts and crosslinking of collagen and elastin in artery walls are governed, in part, by sex hormones; and (3) levels of sex hormones during the neonatal period determine to a large extent mechanical properties of arteries in the human adult. Four approaches are planned: (1) the direct effect of sex steriods on cross-linking of collagen in developing connective tissue will be studied in the rat, using the Ivalon sponge-implant method, with injection of the steriod directly into the sponge. Formation of crosslinks in extractable collagens and degree of crosslinking in insoluble collagen will be studied as a function of the sex hormone injected into the sponge implanted in both sexes and in gonadectomized rats. (2) The effect of sex steroids on the extent of crosslinking of elastin in the rat aorta that occurs during the first days of life and when the animal reaches maturity will be determined. These findings will be related to pressure-volume curves of the artery. (3) The effects of sex steroids administered to the canine neonate on collagen content and crosslinking and physical properties to coronary arteries of the young adult will be studied. (4) The effect of age on collagen crosslinking and collagen and elastin content in rat arteries from birth to 2 years will be related to arterial distensibility. A proven association between circulating sex steroid levels during early life and the ultimate mechanical properties of the artery wall should open up exciting possibilities for attenuating age-related arterial disease in man.